1. Field of the Invention
This invention relates to an improved process for producing pentachloronitrobenzene by the nitration of pentachlorobenzene with concentrated nitric acid.
2. Description of the Prior Art
Pentachloronitrobenzene (sometimes referred to herein as PCNB) is widely used today as a soil fungicide. It is particularly effective in controlling plant diseases caused by botrytis, fusarium, rhizoctonia and anthracnose. However, some commercial products of PCNB have been questioned on environmental grounds because of the presence of relatively large amounts of two comtaminants contained therein. These chemicals, pentachlorobenzene (sometimes referred to herein as PENTA) and hexachlorobenzene (sometimes referred to herein as HCB), have been found to bioaccumulate in the fatty tissue of animals. Therefore, their presence in this desirable soil fungicide may cause a health hazard. For example, foodstuffs produced from soil which has been treated with quantities of impure fungicide may have PENTA and HCB leached into them and, thus, these impurities may accumulate in humans when they are later eaten. Also, cattle and other livestock that graze on treated grass or other pastures or grains may accumulate undesirable amounts of these impurities. And, furthermore, the farmer, when applying PCNB to soil, may breathe in significant quantities of these impurities.
Several methods are now known for the preparation of PCNB. Those of significance to the process described herein involve the nitration of pentachlorobenzene with mixed nitration acid, which consists essentially of a mixture of nitric and sulfuric acids.
In particular, U.S. Pat. No. 4,026,955, issued on May 31, 1977 to Breaux, Newman and Quinnett, teaches one such process. In this patent, it is disclosed that PCNB having reduced HCB content can be produced by a process wherein, first, pentachlorobenzene and mixed nitration acid are mixed together at an initial reaction temperature in the range of 100.degree.-120.degree. C. This resulting reaction mixture is then heated to an intermediate temperature in the range from 130.degree. C. up to the melting point of PCNB and this reaction temperature is maintained for a time period sufficient to deplete the nitric acid concentration to a value of not more than 1% by weight of the reaction mixture. And, then, this reaction mixture is heated to a temperature in the range of 142.degree. C. to 160.degree. C. to form a melt and, finally, this melt is cooled to recrystallize the PCNB product.
Furthermore, U.S. patent application Ser. No. 826,603, filed on Aug. 22, 1977 by Walter Gay, teaches a process similar to that of Breaux et al. In that patent application, it is disclosed that the HCB formation may be substantially reduced by reacting any residual nitric acid with HCl immediately before the above-mentioned melt-forming step occuring in the temperature range of about 142.degree. C. to about 160.degree. C.
While the processes disclosed by these two references represent significant advances in producing relatively high purity PCNB, more recent research has shown that PENTA impurity level can be further substantially lowered by the process of the present invention.